The present invention relates to spindle motors used in applications such as disc drive data storage systems. In particular, the present invention relates to measuring half frequency whirl vibrations that occur in spindle motors.
Spindle motors are commonly used in various applications wherein a precise rotating movement is required. These applications include disc drive data storage systems and their test apparatus (spin-stands). These disc drives and spin-stand testers usually incorporate one or more discs mounted for rotation on a rotor of the spindle motor. Data is recorded and read from a plurality of concentric tracks on the discs by an array of read/write heads. The heads are typically moved radially from track to track on the disc by an actuator assembly.
Advances in disc drive technology have revolved around reducing the size of disc drive components and the size of the overall disc drive. Smaller disc drives can allow for a reduction in overall size of computer systems into which disc drives are installed. With the reduction in size of the disc drive, more space is available within the computer system for other components. In addition to small disc drives, the disc drive industry has also made advances toward increasing the storage capacity of individual disc drive units.
The reduction in size of the disc drive can compound certain problems often associated with various operational features of disc drives. It also places greater performance demands on spin-stands used to test various components of the drive. One such problem involves vibrations or harmonic oscillations in the disc drive and spin-stand tester. The effect of vibrations and oscillations has become magnified as the size of the drive is reduced and data tracks are spaced closer together. As a result, the overall performance of the drive and spin-stand are negatively impacted.
One source of vibration in a disc drive and a spin-stand is from the spindle motors that they employ. These spindle motors typically include a stator comprising a core having windings arranged thereabout and a rotor shaft. Bearings support the rotor shaft in the radial and axial directions, the bearings being lubricated by a fluid. Large amplitude vibration can be caused by imbalance, rotor shaft flexibility, bearing flexibility, fluid film forces in the bearings as the shaft rotates, etc. One particularly common vibration mode occurs at approximately half the shaft rotation frequency. This vibration mode is called half frequency whirl. This half frequency whirl phenomenon is especially prominent in motors that use fluid dynamic bearings. Obtaining precise measurements of half frequency whirl is useful for design verification, quality assurance and failure analysis of spindle motors and disc drives and spin-stand testers which include these motors.
Various direct and indirect measurement techniques have been employed to determine half frequency whirl in a spindle motor. One direct measurement technique used to determine half frequency whirl includes placing a capacitance probe near the outer surface of the rotor shaft of the spindle motor to measure changes in position of the shaft while it rotates. These position measurements from the capacitance probe are fed to a spectrum analyzer that computes a frequency spectrum of the position measurements. The magnitude of the half frequency whirl is obtained from the frequency spectrum. Since the outer surface of the rotor shaft is not perfectly smooth and the resolution of a capacitance probe is relatively low, the half frequency whirl determined from such measurements is imprecise.
One indirect measurement technique for determining half frequency whirl includes utilizing proximity displacement probes to measure the radial motion of an edge of a disc that is mounted on the rotor shaft of the spindle motor instead of directly measuring rotor shaft motion. The half frequency whirl is determined as a function of these measurements. However, due to disc manufacturing and assembling errors, such as imperfection in disc roundness and misalignment between the disc and the rotor shaft, the half frequency whirl determined from such measurements is inaccurate. Other current direct and indirect half frequency whirl measurement techniques have similar disadvantages.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.
An apparatus and method of indirectly measuring half frequency whirl in a spindle motor having a rotor adapted to rotate a disc having a track which is followed by a transducer head that is actuated by a control system is provided. Vibration signals produced at the transducer head while the transducer head follows the track are detected. The half frequency whirl is determined as a function of the detected vibration signals.
Other features and benefits that characterize embodiments of the present invention will be apparent upon reading the following detailed description and review of the associated drawings.